In general, when a lithium secondary battery using an inflammable non-aqueous electrolyte is overcharged, an excessive amount of lithium is deintercalated from a cathode and the lithium is intercalated into an anode, resulting in the precipitation of lithium metal with high reactivity on the surface of the anode. Also, the cathode becomes thermally unstable, and an organic solvent used as an electrolyte is decomposed to cause a rapid exothermic reaction, resulting in ignition and explosion of the battery.
Additionally, polyethylene used as a separator between a cathode and an anode starts to melt at a temperature of 120˜130° C. as the battery temperature increases. Such shrinking of the separator causes the anode and the cathode to be in contact with each other at the edges thereof, resulting in the generation of an internal short. Thus, a local overcurrent phenomenon occurs in the battery, and such overcurrent causes rapid heat emission and an increase in the temperature of the battery. As a result, ignition of the battery occurs.
To solve the above-mentioned problem, Japanese Laid-Open Patent No. 1994-150975 discloses a method of filling an electrolyte into a battery under pressure by using carbon dioxide, so that the electrolyte is easily discharged to the exterior together with carbon dioxide when the battery temperature abnormally increases. However, when the electrolyte is incorporated into the pores present inside a separator or into an electrode, the electrolyte cannot be discharged to the exterior merely by the gas pressure. Therefore, the problem of ignition of a battery caused by the decomposition of an electrolyte is still not solved.
Additionally, Japanese Laid-Open Patent No. 1999-317232 discloses a method for imparting flame resistance to an electrolyte for a lithium secondary battery by introducing a phosphate-based flame retardant, such as trialkyl phosphates, trimethyl phosphate or dimethyl phosphate, into the electrolyte. Such phosphate-based flame retardants are molten by the heat generated upon the ignition of a battery, and surround the surface of an electrode so as to prevent the electrode from being in contact with oxygen, and to serve as a fire extinguishing agent. Therefore, it is necessary to introduce a great amount of flame retardants into the electrolyte, and the use of such a great amount of phosphate-based flame retardants causes degradation of the quality of the battery. Additionally, because an electrolyte with high inflammability causes rapid propagation of fire as soon as it is ignited, the phosphate-based flame retardant alone is insufficient to accomplish fire extinguishment in the battery.